A vortex flow meter provides a frequency output corresponding to the vortical speed (number of vortexes formed) which is proportional to the quantity of air to be measured. A method for injecting a predetermined quantity of fuel into an engine synchronously with the frequency output which is proportional to the quantity of air drawn thereinto has been proposed in Unexamined Utility Model Publication No. 133919/ 1978 or Unexamined patent publication No. 5448/1980. This method has the disadvantage that a lower frequency of fuel injection is likely to bring about changes in rotation during the idling operation of the engine during which only a smaller quantity of air is drawn thereinto, particularly when the engine is of the multicylinder type. On the other hand, a prior method which injects fuel in accordance with the rotatory speed of the engine stabilizes the idling operation, since it enables the uniform supply of fuel to all the cylinders even during the idling operation during which only a smaller quantity of air is drawn into the engine. The method of injecting fuel in accordance with the frequency output proportional to the quantity of air drawn into the engine, however, improves the acceleration performance of the engine, since it enables the injection of fuel without any appreciable delay when the quantity of air has been changed. On the other hand, the method of injecting fuel in accordance with the rotatory speed of the engine lowers its acceleration performance, since an increase in its rotatory speed takes place with a certain amount of delay after the quantity of air drawn thereinto has been changed.